There are six constraints which are NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, DEFAULT. I will explain all of them with giving example for each one.
NOT NULL : If you use this constraint for a column then this column cannot contain null value.
UNIQUE : Ensures that every column has a unique value.
PRIMARY KEY : Primary key has two specific rules If a column has primary key constraint then it must be not null and unique. When we use primary key it will help us to find record which we are searching correctly.
FOREIGN KEY : This constraint ensures the value in a column in a table to match value in a column in another table.
CHECK :Ensures which the value in a column meets a specific condition.
DEFAULT : It is using for put a default value when create new record for a column.